Would say yes. We have an old Vokera big thing in our loft. Had them out about 10 times a coupla winters ago for a hardto fix thermostat problem. I do think they should have fixed it in the first two or three times they were out but none the less, it was fixed eventually and didnt cost us a penny..
No doubt would have cost a fortune otherwise..

I've used them on my boiler for a few years - been worth it's weight in gold! Repairs are 'free' as that's what you're paying for. Breakdowns will cause your premium to go up, but even then it's been good value (heat exchanger was the last thing we got changed out, not a cheap job)

Emergency breakdown cover on home insurance is often cheaper and covers more stuff, however whenever I've had to call them out (not British Gas) it takes them 2 days to send an engineer out to confirm that I need a new Thermistor and then another 2 / 3 days to get and fit it.

It will cost a fortune either way: Multiply your BG cover premium by say 15 years (the lifespan of a boiler)and you're looking at 2 grand.
Having said that we would have got our moneys worth as we've had 3 new fans and a diverter valve over 11 years (replaced them myself!).

No save your money and engage the services of a Gas engineer who specialises in your make when/ if your boiler goes faulty.

BG take ages to send someone out who inevitably wont be trained or kitted to repair anything other than a simple fault. You'l wait days for parts and then they will eventually fix.

Sisters boiler went faulty last week. so far they have taken 4 days to come and tell her that they think its the expansion tank and that they'll need to get a part. Living in Scotland you need heating and hot water at this time of year.

I called the loacl saunier duval engineer and it was fixed that day as he had parts on his van.

Like all insurance. Worth it if you need to claim ! Personally it saved me a lot of money when our system went wrong a couple of years ago. And seem to need a motorised valve or something most years, that can justify the premiums on its own.
EDIT: in addition I have found their response times very quick usually next day.